This invention concerns a process for manufacturing pesticides from methyl isocyanate (MIC) in which the MIC is generated as a gas and consumed in a close-coupled reactor column so that very little MIC is present at any time. The use of MIC to manufacture pesticides is known, but all of the known methods involve storing and handling liquid MIC. This is dangerous because MIC is extremely toxic and very reactive (even with water) in liquid form. By the method of this invention, a gaseous stream containing MIC and water, typically one generated by controlled oxidative dehydrogenation of relatively nontoxic nonreactive monomethylformamide (MMF), is passed into a column reactor where the MIC reacts with an oxime or phenol to form the carbamate.
Heretofore, prior art methods directed to minimizing intermediate reactions of the MIC/water formed by oxidizing MMF featured various methods of physically separating the MIC from the water. Such methods typically involved the intermediate isolation and storage of MIC preparatory to its reaction with the oxime or phenol to form the desired pesticide. The process of this invention eliminates the need for storing MIC with the consequent risk. The MIC is contacted with the oxime or phenol continuously as the MIC is being made, in the vapor phase, in a closed system.
U.S. Pat. No. 3,576,834 discloses certain N-methyl carbamates and their preparation by the reaction of an oxime with methyl isocyanate (MIC) in an aprotic solvent. The MIC used in that method can be prepared in a variety of ways, one of which is the gas phase reaction of N-methylformamide with oxygen, as shown in U.S. Pat. No. 4,207,251. That patent discloses physical separation of the water and MIC to avoid reaction between the MIC and the water. Japanese Kokai 56/100751 discloses a process for making MIC from monomethylformamide and that the water by-product of the reaction can be separated from the MIC by ordinary distillation.
U.S. Pat. No. 3,506,698 discloses making certain N-methyl carbamates by contacting an oxime with MIC in a reaction medium that comprises at least 50% water. The disclosed reaction takes place in the liquid phase. In several Examples, the reactants are added to water and the reaction mass is cooled, apparently to insure reaction within the preferred temperature range of 20.degree. to 60.degree. C.
U.S. Pat. No. 4,207,251 describes the problem caused by reaction between various isocyanates and water produced by oxidation of formamides. The disclosed solution is to physically separate the isocyanate and water to prevent them from reacting. Disclosed separation methods include filtration, extraction, use of molecular sieves, water-absorbing agents or a water-immiscible solvent for the isocyanate. This patent is silent concerning conditions under which isocyanates are reacted to form pesticides.